Leishmania organisms are intracellular protozoan parasites of macrophages that cause a wide range of clinical diseases in humans and other animals. The pathological outcomes range from simple self-healing cutaneous lesions in cutaneous leishmaniasis (CL) to acute visceral leishmaniasis, commonly referred to as kala-azar. Symptoms of visceral disease include fever, emaciation, hyper gammaglobulinanemia, hepatosplenomegaly, and pancytopenia. Although L. tropica generally causes cutaneous leishmaniasis, a variant form of visceral disease caused by L. tropica has been noted. Exposure of a yet unknown number of individuals during the Gulf War to L. tropica has resulted in a variant form of visceral disease, referred to as viscerotropic leishmaniasis (VTL). Symptoms associated with VTL differ somewhat from classical visceral disease in that some patients lack both fever and hepatosplenomegaly but have chronic weakness and malaise. More importantly, serum antibody titers to Leishmania are significantly lower than those observed in patients with classical visceral leishmaniasis (VL).
The latency period observed with leishmanial infection can be extensive, with asymptomatic periods of greater than ten years not uncommon. Occasionally, parasitic infection is detected only when the individual has entered an immunocompromised state. This extended latency period increases the risk of further transmission of the parasite. There are documented cases of transfusion-acquired leishmaniasis, even from asymptomatic individuals. In addition, experiments have demonstrated the survival of L. tropica parasites in blood products during storage for up to a month.
Early diagnosis of leishmaniasis is critical for successful treatment but is difficult to achieve with existing techniques. The recent experience of leishmaniasis associated with Desert Storm has underlined a fundamental problem in the diagnosis of one of the world's most widespread protozoal infections; there are no simple, effective, and reliable diagnostic tests. Diagnosis of classical VL has exploited the elevated antibody response to parasite antigens in tests involving serological reactivity to whole or lysed promastigotes or to recombinant antigens. Confirmation of infection is generally made by the isolation of live parasites from spleen, liver, bone marrow, or lymph nodes. However, these tests are not without disadvantages. Tests relying on the use of whole parasites or crude lysates are difficult to standardize. In addition, preparation of the test antigens is expensive and difficult on a large scale.
Diagnosis of leishmaniasis in VTL patients is generally unsuccessful using current test strategies. In particular, the serological test antigens are generally disrupted promastigotes, and in VTL patients, antibody reactivity to these antigens is low. Thus, currently available methods often fail to detect this potentially fatal disease early enough to allow effective treatment.
In addition, currently there are no vaccines against Leishmania, in spite of the public health importance of this disease. Protective immunity against Leishmania is mediated by T cells, and Th1 cells in particular. An antigen capable of stimulation of such protective T cells would be a vaccine candidate.
Accordingly, there is a need in the art for more sensitive and specific methods for detecting and protecting against Leishmania infections resulting in VTL. The present invention relies on immunodominant antigens of Leishmania that allow detection of both humoral and cellular immune responses, while providing other related advantages.